Regulating Charge Injection in Ambipolar Organic Field-Effect Transistors by Mixed Self-Assembled Monolayers Yong Xu, †,∥ Kang-Jun Baeg, ‡,∥ Won-Tae Park, † Ara Cho, † Eun-Young Choi, § and Yong-Young Noh* ,† † Department of Energy and Materials Engineering, Dongguk University, 26 Pil-dong, 3-Ga, Jung-gu, Seoul 100-715, Republic of Korea ‡ Nano Carbon Materials Research Group, Korea Electrotechnology Research Institute (KERI), 12 Bulmosan-ro 10 Beon-gil, Seongsan-gu, Changwon, Gyeongsangnam-do 642-120, Republic of Korea § Nuclear Fuel Cycle Development Group, Korea Atomic Energy Research Institute, 1035 Daedok-daero, Yuseong-gu, Daejeon 305-353, Republic of Korea * S Supporting Information ABSTRACT: We report on a technique using mixed self- assembled monolayers (SAMs) to finely regulate ambipolar charge injection in polymer organic field-effect transistors. Differing from the other works that employ single SAM specifically for efficient charge injection in p-type and n-type transistors, we blend two different SAMs of alkyl- and perfluoroalkyl thiols at different ratios and apply them to ambipolar OFETs and inverter. Thanks to the utilization of ambipolar semiconductor and one SAM mixture, the device and circuit fabrications are facile with only one step for semiconductor deposition and another for SAM treatment. This is much simpler with respect to the conventional scheme for the unipolar-device-based complementary circuitry that demands separate deposition and processing for individual p-channel and n-channel transistors. Our results show that the mixed-SAM treatments not only improve ambipolar charge injection manifesting as higher hole- and electron-mobility and smaller threshold voltage but also gradually tune the device characteristics to reach a desired condition for circuit application. Therefore, this simple but useful approach is promising for ambipolar electronics. KEYWORDS: ambipolar, charge injection, conjugated molecules, organic filed-effect transistors, self-assembled monolayer 1. INTRODUCTION Solution processable π-conjugated molecules are promising to compose organic devices for wide applications in large-area, flexible, transparent, environmentally friendly, and low-cost electronics. 1−3 As a core feature of those achievements, unconventional deposition and patterning techniques especially solution-based printing will shift semiconductor manufacturing to a new paradigm by replacing the present costly photo- lithography and vacuum-based processes. 4 Printed organic field- effect transistors (OFETs) that stand for the primary building blocks of numerous expected applications have been intensively studied. 5,6 At the beginning of OFET research, for the sake of simplicity and accessibility most of devices were built on doped Si wafer covered with thermally grown SiO 2 that serve as the gate electrode and gate dielectric, respectively. Such a rigid substrate limited the flexibility of the composed circuits even if devices themselves would be quite soft. Moreover, SiO 2 contains high density of hydroxyl groups known as electron traps significantly suppressing n-type device characteristics thereby the majority OFETs so far has been p-type transistors. 7 This unipolarity impedes the interesting OFET applications using ambipolar transport characteristics in organic semi- conductors (OSCs). In fact, there are plenty of OSCs having ambipolar properties, 8 in particular, the recently reported donor −acceptor copolymers (e.g., diketopyrrolopyrrole dithienylthieno[3,2-b] thiophene (DPP-DTT)) that exhibit small bandgaps and unprecedentedly high carrier mobility up to 10 cm 2 /(V s). 9 In order to trigger the intrinsic nature of ambipolar transport in OSC, researchers started to use the siloxane-based polymer dielectrics (e.g., benzocyclobutene (BCB)) or capping layers of alkyl-silane self-assembled monolayers (SAMs), 10 and since then, ambipolar related applications were extensively investigated. 1 For instance, Zaumseil et al. demonstrated light-emitting OFETs based on ambipolar polymeric semiconductors. 11 Ambipolar OFETs are appealing for complementary metal- oxide-semiconductor (CMOS) organic integrated circuits (ICs) as they can much simplify circuit design and greatly improve circuit performance with respect to the present unipolar device- based CMOS ICs. 12 On the other hand, the conventional Si- Received: June 14, 2014 Accepted: August 5, 2014 Research Article www.acsami.org © XXXX American Chemical Society A dx.doi.org/10.1021/am5037862 | ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX